Dick_R Tracking C Flow through Microbial Communities

An Unrecognized Pathway for C and N Stabilization in No-Tillage Agroecosystems

ABSTRACT

Global industrialization has enriched levels of atmospheric carbon dioxide (CO2) contributing to climate change. The total amount of carbon (C) present in the atmosphere passes through the terrestrial biomass within about 6-7 years and C stored in soils as soil organic matter is two times greater than that of the atmosphere and almost three times larger than that of the aboveground biomass. Therefore, there is considerable interest in storing C in soils to offset elevated atmospheric CO2 levels. Agricultural land being highly managed, offers potential to sequester C and a key management factor is tillage that affects the partitioning of C between respired CO2 and sequestered in soil organic matter. Habitual soil tillage increases C losses from soil through incorporation of residues. These losses have left many agricultural soils relatively C depleted but this could be undone through improved soil management.

No tillage (NT) system that leaves crop residues on the field and gives minimal soil disturbance is thought to be a strategy to maximize C sequestration in croplands over conventional tillage (CT) system that disturbs the entire soil surface and is performed prior to and/or during planting. Largely unsubstantiated conventional thinking suggests that NT stabilizes C by surface decomposition and binding of organic C to mineral soil layers.

To date, NT has received less scientific and public attention than it deserves in agricultural lands of Turkey and CT is the common tillage system applied to the majority of agricultural soils characterized by poor organic matter content especially under dryland conditions in central Turkey. However, the experiments on NT in the central Turkey indicated that this technology could be well-adapted to the dryland conditions and be more practical and economical. Farmers have been shifting from tillage intensive production systems to no-till production systems with about 23% (over 25 million hectares) of U.S. agricultural land which is being managed with NT practices. This has been advocated as a strategy to simultaneously mitigate increasing atmospheric C levels, and maintain or improve soil quality.

On the other hand, to effectively manage NT practices fundamental information is needed on how these systems sequester C. The microbial communities in concert with soil fauna are central components of soil controlling decomposition and the concomitant partitioning of C back to the atmosphere as CO2 or storing in soils. Microorganisms ultimately break down all plant biopolymers into their monomeric structural units which then react with nitrogen-containing compounds to form dark-colored complex, high molecular weight polymers. These humic substances are the recalcitrant forms of soil organic matter that are expected to play a significant role as a C sink regulating CO2 levels. Many fungi possess extensive enzyme systems that can attack a range of compounds from relatively more simple carbohydrates to more complex proteins, cellulose, and lignin. Saprotrophic fungi play a central role in decomposition at all stages of decomposition. Furthermore, fungi, particularly, some microscopic fungi play a significant role in the synthesis of humic substance in soils. These fungi degrade lignin and cellulose and in the process synthesize appreciable amounts of humic acid-like polymers.

In light of these facts and considerations, our proposed research will study a largely unrecognized mechanism – namely fungal transport of C and N from the NT litter layer to the mineral soil layer. We hypothesize that reciprocal transport of energy and nutrients between surface litter and mineral soil is an important mechanism generated by fungal hyphae. To isolate fungal transport over leaching or soil faunal activity we will utilize the novel phospholipid fatty acid-13C tracking method (PLFA-13C) in combination with manipulations of fungal barriers in field experiments. Stable isotope probing with 13C of specific biomarkers, such as PLFAs, is a promising approach to directly link specific microbial processes with the organisms responsible; thus shedding valuable insight into the biogeochemical transformations in natural environments that are mediated by microorganisms.

Our study will be based on tracking 13C into PLFAs by field incubation/decomposition technique utilizing 13C labeled plant litter residue which can be made possible by successfully using a pulse-chase 13C labeling method that results in a uniform label of plant residue. Utilizing this labeled material allows us to monitor the diversity of organisms involved in decomposing and cycling of C by tracking the total number of phospholipid peaks occurring during different time periods of decomposition. Moreover, during 13C labeling process, plant seedlings will also be applied with labeled nitrogen (15N) as urea thus enabling us to determine the potential for transport of N via fungal activity from litter layer to mineral soil and also uptake of N by plant.

The most important issues to be dealt with in this research are: (i) which microbial communities (ie: bacteria vs. fungi) dominate the decomposition of surface residues and transformation of surface residue C into recalcitrant soil C in no-till and plow till soils; (ii) whether there are differences in biologically mediated translocation of surface residue C between no-till soils and plow till soils; (iii) which soil C pools deposit the decomposition products of 13C labeled surface residues in North-Central Ohio soils managed with no-till or plow till practices (iv) whether saprotrophic fungi transport mineralized C from surface residues to mineral soil through their hyphal networks; and moreover (v) what is the role of soil fauna in contributing C mineralization and microbial decomposition of litter, and C sequestration in mineral soils.

When above-mentioned issues are resolved, we will be in a position to develop management systems that optimize hyphal transport of C mineral soil. This could include approaches such as fertilizer management and breeding for crop residues that promote fungal transport and deposition of litter derived humic substances that result in stable C pools. The long-term goals of this project are to develop crop residue and soil management systems that optimize microbial sequestration of C in soils to mitigate global climate change and more importantly to adapt the experiences and knowledge obtained from this project to the soil management systems in Turkey.